1. Field of the Invention
This invention relates to hydrophilic plastic materials useful as soft contact lenses, and more particularly relates to treatment of said hydrophilic soft contact lenses to make them more resistant to clouding and discoloration in use.
2. The Prior Art
It has long been known to fabricate contact lenses from rigid, clear plastic materials, e.g., the essentially hydrophobic acrylic-type polymers as polymethyl methacrylate and the like. Although more safely handled and used than glass, such hydrophobic plastic materials have been only moderately successful as contact lenses, oftentimes being too hard and uncomfortable to the wearer with continued use. More recently, a different type of contact lens known as a soft lens has been developed from hydrophilic polymers. This lens is more easily accommodated by the eye and has become increasingly important in ophthalmological practice.
Hydrophilic soft contact lenses, in general, are fabricated from lightly cross-linked, essentially water-insoluble homopolymers, copolymers, terpolymers, etc., derived from one or more monomers containing hydroxyl groups which impart to these polymers their affinity for water. Hydrophilic polymers may further be defined as coherent, three-dimensional polymer structures or networks which have the ability to absorb or imbibe water, even in large quantities, e.g., up to 90 weight percent, without dissolution. When containing water in any amount whatsoever, a hydrophilic polymer will expand correspondingly to a hydrophilic polymer "gel." More specifically, a hydrophilic polymer containing imbibed water is known generally in the art as a "hydrogel." At the present time, the specific class of hydrated hydrophilic polymers, i.e., hydrogels which enjoy particular commercial acceptance as soft contact lenses, are broadly classed as the acrylic ester-type. In the main, acrylic ester hydrophilic polymers are derived by polymerizing a water-soluble monoester of acrylic or methacrylic acid in which the ester moiety contains at least one hydrophilic group in the presence of a minor amount, e.g., 5 percent or less by weight, of a bifunctional monomer, e.g., a diester of acrylic or methacrylic acid. The bifunctional material cross-links the hydrophilic group-containing monomer as it polymerizes. A specific acrylic ester hydrophilic polymer which has enjoyed significant commercial success as a soft contact lens is composed of polyhydroxylated alkyl, e.g., ethyl methacrylate (HEMA), cross-linked with the difunctional monomer ethylene glycol dimethacrylate.
Although contact lenses derived from hydrophilic polymers are much softer than the earlier hard contact lenses and can thus be accommodated by the wearer with much less discomfort, they nevertheless have disadvantageous properties and likewise have not been completely satisfactory. These lenses can be contaminated with pathological bacteria and fungi on their surfaces. If the lenses are not regularly cleaned and disinfected, or if stored in contaminated solutions, the pathogens can then be easily sorbed by the lens material due to its flexible, hydrophilic polymer structure. Corneal-damaging chemical residues from improper cleaning and/or disinfecting techniques can likewise be introduced into the lens material in this way. Proteins and other substances normally in the eye environment can be easily diffused into the lens with use, likewise because of its flexible, hydrophilic polymer structure. These substances cause discoloration and clouding of the lens with the repeated cleaning and disinfecting techniques practiced by the wearer. Further, the lenses oftentimes can lose sufficient amounts of water during use to deleteriously affect their dimensional stability and optical acuity.
It is an object of this invention to provide a soft contact lens wherein the diffusion of protein and other migratory eye substances is significantly inhibited so as to lengthen its life and optical effectiveness.
It is a further object of this invention to provide a soft contact lens which will retain a sufficient quantity of water during use to maintain its dimensional stability and optical acuity.
These and other objects of this invention will become apparent to those skilled in the art from the description of the invention which follows.